Millimicrosecond pulse circuits



Aug. 16,1960 J.A.NARUD 2,949,578

MILLIMICROSECOND PULSE CIRCUITS Filed Jan. 29, 1957 2 Sheets-Sheet 1 INV EN TOR.

Aug. 16, 1960 J. A. NARUD 2,949,578

MILLIMICROSECOND PULSE CIRCUITS I Clan Arne /Varud BY y( i du 0 WW 4 rrom/E Ys lnited States llatent Zdg Patented Aug. 16, i960 iceMILLIMICROSECOND PULSE CIRCUITS Jan A. Narnd, Concord, Mass., assignerto the United States of America as represented by the Secretary of theNavy 'Filed llan. 29, 1957, Ser. No. 637,051

4 Claims. (Cl. sns- 34) The present invention relates generally toelectronic pulse circuits and, more particularly, to apparatus for andmethods of generating, discriminating and counting millimicrosecondpulses.

In the fields of nuclear instrumentation, computers, automatic controls,etc., the need has recently arisen for electronic pulse circuits havingresponse times in the range of approximately one to iiftymillimicroseconds. Conventional circuits which use multi-electrodethermionic tubes cannot be effectively employed in this range primarilybecause the relatively high interelectrode capacitances of these tubestend to round oi the sharp corners of any very narrow pulses present,delay their steep wave fronts slightly and broaden and reduce theiramplitudes. Although the interelectrode capacitance can be reduced bymodifications of the tube structure, these changes unfortunately areusually accompanied by a reduction in the tubes transconductance. Sincethe resolving time of a flip-hop pulse generating circuit, such as, forexample, the multivibrator, is roughly proportional to the totalcapacitance to ground of a tube divided by its average transconductance,no appreciable net improvement in the response time is realized byredesigning the thermionic tube.

Although the phenomenon of secondary emission has been used successfullyfor a long time to amplify the minute photoemission current inphotoelectric tubes, its application to thermionic tubes has laggedbehind. The main reason for this lag is to be found in the difcultyencountered in developing a coating with a high secondary emission ratiofor the secondary cathode that does not evaporate or become contaminatedat the relatively high temperatures existing in such tubes. The averagenumber of electrons released from a surface by each incident or primaryelectron is known as the secondary-emission ratio. Certain combinationsurfaces, such as alkali halides on an alkali metal base and alkalioxides on various metal bases, have been developed recently which canwithstand these high temperatures and give secondary emission ratios ashigh as eight per primary to eleven per primary, as contrasted with puremetals where the ratio is in the vicinity of one.

Due to the multiplying action of secondary emission, this type of tubehas a maximum plate transconductance from four to eight times largerthan what can be expected of regular pentodes. Also, a relatively smallchange in grid voltage can bring the tube from cutoff to saturation.These characteristics plus the fact that the ratio of the dynode currentor the plate current to the associated electrode capacitance is veryhigh for these tubes make them particularly advantageous in circuitswhere fast switching action is required.

Furthermore, since the grid voltage of a secondary emissive type tube isin phase with the dynode voltage, positive feedback action can simply beobtained by intercoupling dynode and grid together through a feedbacknetwork directly. In other words, no extra signal delay is added in thiscase by having to go through an extra tube for phase inversion. Thus,this tube makes it possible to have a feedback arrangement with theminimum amount of loop delay, thereby making ideal for high speedswitching action and as a generator of short pulses.

Accordingly, it is a primary object of the present invention to providean electronic pulse generator employing a secondary emissive tube in itscontrol circuit for generating millimicrosecond pulses having extremelyshort rise times.

A secondary object of the present invention is to provide adiscriminating circuit capable of rejecting rnillimicrosecond pulseswhose amplitudes do not exceed a predetermined magnitude.

A further object of the present invention is to provide a pulsegenerator capable of producing pulses having a rise time ofapproximately six millimicroseconds and a variable width fromapproximately iifteen millimicroseconds and up at extremely highrepetition rates.

A further object of the present invention is to provide a circuitcapable of shaping millimicrosecond pulses.

A still further object of the present invention is to provide a countingcircuit capable of registering millimicrosecond pulses and having `aresolving time of approximately twenty millimicroseconds.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings, wherein:

Fig. 1 illustrates an electronic pulse circuit capable of performing aseither a millimicrosecond pulse amplitude discriminator or amillimicrosecond pulse generating circuit; and

Fig. 2 illustrates a circuit arrangement which can be employed to countmillimicrosecond pulses such as those produced by the above circuit.

Referring now to Fig. 1, which shows a preferred embodiment of theinvention, and taking the case where the circuit is to perform as amillimicrosecond pulse generator, input pulses of suitable amplitude andrise time, such as, for example, those produced by a freerunningmultivibrator, are coupled to the control grid of a secondary emissiontype amplifying tube 5. A differentiating network consisting of a seriescapacitor l and a shunt resistor 2 is included in the input circuit ofthis amplifying tube to prevent signals having a slow decay from causingspurious operation of the circuit. Amplitying tube 5 is normallymaintained nonconducting by a negative bias applied to its control gridvia the moving Contact of potentiometer 3 connected between a negativevoltage source 26 and ground. The amplitude of the trigger signal isselected so that the positive trigger pulse obtained by diierentiatingthe leading edge of the pulse has suliicient amplitude to overcome theblocking bias and drive tube 5 to conduction. To improve the wave formof the negative pulse thus developed at the plate of tube S, a peakinginductance 7 is inserted in series with load resistor 6.

At the same time that this negative pulse occurs at the plate, a similarpositive pulse is generated at dynode 4. This is due to the fact thatupon electron bombardment the dynode loses more electrons than itreceives from the cathode due to secondary emission. This positive pulseis directly coupled via capacitor 18 to the control electrode of asecondary emissive tube 20 lwhich is also normally maintainednonconducting by the negative voltage applied to its control grid fromthe juncture of series resistors i3 and 14 connected between thenegative voltage source 26 and ground. By coupling the dynode 25 to oneside of the capacitor 18, positive feedback is obtained and this tubeperforms as a monostable multivibrator having a stable and an unstableoperating point, the tirst at cutot and the other at saturation, and anadditional unstable operating point at an intermediate level of tubeconduction. These various operating points are determined primarily bythe dynode current versus dynode voltage characteristic of tube 20 andthe magnitude of the parallel connection of dynode resistor 1G and thegrid resistors 17 `and 15.

It is well known that if such a multivibrator rests at its permanentlystable operating point` an instantaneous transition to the other stableoperating point can only take place when the applied trigger signal,which in this case is the positive pulse obtained from dynode 4, hassuiiicient magnitude and width to slide the circuit past the unstablepoint. If the amplitude of the trigger signal is insuicient, the tubereturns to the permanently stable operating point from whence itinitially started.

In the present case, the positive pulse at dynode 4 has suiiicientamplitude to bring about this transition and, as a consequence, tube 2t)is rapidly driven to saturation by the regenerative action of capacitor18 and the leading edge of a negative output pulse appears in the platecircuit of this tube at terminal 24.

To increase the trigger sensitivity of the pulse generator further, thenegative signal appearing at the plate of tube 5 is also coupled viacapacitor 9 to the cathode of tube 20. This cathode is connected to`ground through a diode 19 in order to make the impedance between thecathode and ground large when the trigger signals are applied and smallwhen tube 20 becomes conducting. In this way very rapid and precisetriggering of tube 20 is obtained. Once tube 20 is driven to itstemporary stable operating point, the circuit will stay in the vicinityof this point until the condenser 1S has accumulated enough charge sothat the loop gain of the circuit becomes equal to unity. A jump back toa point beyond the permanent- `ly stable operating point now takes placeafter which the circuit again comes to rest. This process results in apositive pulse shaped behavior of the voltage at the dynode and anegative one at the plate. The time it takes for the circuit to go fromthe temporary stable operating point to the point of unity loop gaindetermines solely the width of the output pulse. This time, in turn, isdetermined by the magnitude of resistors 10, and 17, condenser 18, thegrid bias, the location of the permanent and temporary stable operatingpoints and the unity loop gain point. Therefore, the pulse width may bevaried by making any of these quantities variable. As a practical point,however, it is usually easier to vary the grid-resistor 13, condenser 18and the grid bias. Since varying the grid-resistor 15 and condenser 18gives better linearity, these are usually used as the main pulse widthcontrol while the grid bias is merely varied for adjustment purposes.

In the case where the circuit of Fig. l is to perform as amillimicrosecond pulse amplitude discriminator, the discriminating levelis set by adjusting the location of the movable tap of potentiometer 3.Only those input pulses whose amplitudes exceed the magnitude of theblocking bias thus supplied to the control grid of amplifying tube 5will succeed in reproducing corresponding negative pulses at terminal24.

Fig. 2 illustrates a counting circuit for registering a sequence ofmillirnicrosecond pulses during an interval of time up to a hundredmicroseconds. In other words, it is a kind of fast memory circuit thatis able to store information coming in at a fast rate for a certaininterval of time after which the information stored is destroyed. Toprevent misiire due to input signals having different wave forms, itshould be driven by a discriminator circuit of the type shown in Fig. l.When a positive pulse is applied to the grid of tube 33, which isnormally cut off by virtue of the negative bias on its grid,

a positive pulse appears at the dynode of this tube and a negative oneat its plate. These two electrodes are connected to the dynode ofanother secondary emission tube 38 via the diodes 36 and 35respectively. These two diodes receive a bias from the attenuatorsconsisting of resistors 46, 48, 49 and 52, -51, 53 in such a way thatdiode 36 is conducting when tube 38 is cut off and diode 35 islconducting when tube 38 is conducting. The tube 38 is normallynonconducting by virtue of a negative bias on its grid supplied by theresistor 43 which is connected to the negative voltage supply 44. Also,it has a condenser connected between its dynode and its grid so thatregenerative action may be created in the tube in such a way that it cantemporarily have two stable operating points separated by an unstableone. Hence, if a positive pulse is initially applied to the input of thecircuit, a positive pulse will appear on the dynode of tube 38 sincediode 36 is conducting and diode 35 initially cut olf. This pulse willnow turn tube 38 on and if no other pulses are applied to the circuittube 38 will remain conducting for a time determined mainly by the sizeof the condenser 37 and resistor 54 and 53. When tube 3S becomesconducting, the bias on diodes 35 and 36 will now be such that diode 35becomes conducting and diode 36 cut oli'. Hence, if a second pulse isapplied to the circuit, the negative pulse at the plate of tube 33 willnow pass over to the dynode of 38. Since this pulse is negative, it willthen turn the tube off. Therefore, if the size of condenser 37,resistors 54 and 42 are made large enough so that tube 3S can remainconducting for a long time interval, tube 38 can be turned otr and on bysequences of fast pulses during this interval. Thus, tube 38 will act asa binary scaler being turned on and off by successive pulses applied toits input. In the plate of tube 38 a differentiating network is insertedconsisting of essentially a condenser (41), a diode 60 and a resistor(40) connected up in such a way that when the tube initially turns on nopulse appears at its output while when the tube turns oi a positivepulse will appear. it will be understood, of course, that diode 69 actsas a clamp across load resistor 61 to insure the production of positiveoutput pulses only. Thus, for a succession of pulses applied to theinput of the circuit a positive pulse will therefore appear at theoutput for each second pulse applied and a scale o 2 is achieved.

Obviously many modications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as speciiically described.

What is claimed is:

1. A millimicrosecond pulse generating circuit comprising, incombination, a secondary-emissive tube having as components thereof atleast a cathode, control grid, dynode and anode, a diode connectedbetween said cathode and a reference potential, means for biasing saidcontrol grid negatively with respect to said reference potential wherebysaid tube is normally maintained nonconductive, means -for coupling saidanode via a load resistor to a voltage source positive with respect tosaid reference potential, a coupling capacitor interconnected betweenthe dynode and control grid of said tube for providing a positiveregenerative action therebetween, and means for simultaneouslytriggering said tube by applying a positive pulse and a negative pulseto the control grid and cathode, respectively, of said tube, saidtrigger pulses cooperating to move the operating point of said tube fromits first stable point at tube cutoil" through its intermediate unstableoperating point to its second stable operating point at plate currentsaturation whereby a negative going output pulse having an extremelyfast lrise time is Vgenerated at the anode of said tube, and means forvarying the time constant of the discharge path of said couplingcapacitor thereby to determine the time at which said tube returns toits first stable .5 operating point and the trailing edge of said outputpulse.

2. A millimicrosecond pulse generating circuit comprising, incombination, a secondary electron emissive vacuum tube having at least acathode, a control grid, a dynode and an anode, a coupling capacitorconnected between the dynode and the control grid of said tube so as toprovide an extremely short time constant positive feedback paththerebetween, means for connecting said control grid to a negativepotential so as to normally maintain said tube nonconducting, a loadresistor connected between said anode and a positive voltage, a diode,said diode being connected to said cathode and a source of referencepotential and poled to pass positive pulse from said cathode to saidreference potential, means for coupling simultaneously a positivetrigger pulse to the control grid of said tube via said couplingcapacitor and a negative trigger pulse to the ungrounded side of saiddiode whereby said tube is immediately driven to a condition of platecurrent saturation and whereby the leading edge of a negative outputpulse is developed at the anode of said tube and means for controllingthe time constant of the discharge path of said capacitor so as toestablish the time at which said tube returns to its originalnonconducting state and the time at which the trailing edge of saidoutput pulse occurs.

3. A millimicrosecond pulse generator comprising, in combination, aiirst secondary-emissive tube having as components thereof a controlgrid, a dynode, a cathode and an anode, means yfor applying a blockingbias of a predetermined magnitude to the control grid of said tube formaintaining said tube normally nonconducting, means for applyingpositive input trigger pulses to the control grid of said rst tubethereby to render said tube conducting whenever the amplitude of saidtrigger pulses exceeds said blocking bias, said rst tube developingcoincident positive and negative pulses at its dynode and anode,respectively, whenever it is rendered conducting, a secondsecondary-emissive tube, said second tube having as components thereof acathode, a control grid, a dynode and an anode, means for normallymaintaining said second tube nonconducting, a load resistor connectedbetween the anode of said second tube and a positive voltage, a diode,said diode being connected between the cathode of said second tube and areference potential, said `diode being poled to pass only positivepulses from said cathode to said reference potential, means forinterconnecting the anode of said rst tube to the cathode of said secondtube via a coupling capacitor, means for directly interconnecting thedynodes of said first and second tubes and a capacitor coupled betweenthe dynode of said second tube and the control grid of said second tube,said capacitor providing a short time constant positive feedback pathfor said second tube whereby said second tube performs as a monostablepulse generator when simultaneously triggered by the coincident positiveand negative pulses coupled to its control grid and cathode,respectively, from said first tube generates a negative going outputpulse across said load resistor.

4. In a pulse generator as dened in claim 3, means for adjusting thedischarge time constant of the capacitor coupled between the dynode ofsaid second tube and the control grid of said second tube, thereby toregulate the duration of said negative going output pulse.

References Cited in the file of this patent UNITED STATES PATENTS2,293,177 Skellett Aug. 18, 1942 2,294,782 Jacobsen Sept. 1, 19422,369,631 Zanarini Feb. 13, 1945 2,509,998 Mark May 30, 1950 2,597,796Hindall May 20, 1952 2,847,565 Clapper Aug. l2, 1958 FOREIGN PATENTS221,125 Switzerland Aug. 1, 1942

